1. Field of the Invention
The present invention relates to a hydrostatic system and in particular to a system having a means for monitoring the charge pump flow to ensure adequate charge flow is provided to the hydrostatic pump.
2. Description of the Related Art
A hydrostatic drive utilizes fluid at high pressure and relatively low speed to transfer energy in a closed circuit between a hydrostatic pump and motor. The raise in pressure of the fluid, which will not compress, is what transfers the energy. A charge pump is used to supply fluid, usually oil, under pressure to the hydrostatic pump. An adequate supply from the charge pump is required for proper functioning of the hydrostatic system.
When a hydrostatic system is used to steer a vehicle, such as a track laying vehicle, it is desirable to monitor the flow of the charge pump to ensure that adequate flow is provided for proper steering functioning. Over time, the charge pump may wear, causing a decrease to occur in the charge flow. In a hydrostatic steering system, fluid flow is only required during a relatively small portion of the vehicle operating time, when the vehicle is being turned.
One attempt to monitor the charge flow has been to sense the charge pressure. However, since charge flow is only needed during turning, even if the pump has worn, there will likely be enough charge flow for the charge pressure to be adequate. That is, until steering is needed and the charge flow is now inadequate and the pressure drops. Thus, a pressure sensor at the charge flow outlet is only adequate to detect a catastrophic and sudden failure of the charge pump.
The present invention provides a system for monitoring the charge flow so that a decrease in the flow rate, even if the charge flow pressure remains satisfactory, can be detected. In a hydrostatic system, the case drain flow from the pump and the motor are joined and directed to the reservoir. The charge pump flow and the drain flow are thus equal. The system of the present invention provides a sharp edge orifice restriction in the drain, creating a back pressure in the drain that is proportional to the fluid flow through the drain. A pressure switch mounted in the pump case, or in the drain, detects the back pressure in the drain. The pressure switch is designed to be activated at a back pressure that corresponds to the intended charge flow at a specific engine speed. During operation, if the back pressure is not sufficient to cause the pressure switch to be activated at the predetermined engine speed or at a faster engine speed, the charge flow is not sufficient and a warning signal is generated.